Added ComputedPropertyNames feature

Added [Computed Property Names](http://wiki.ecmascript.org/doku.php?id=harmony:object_literals#object_literal_computed_property_keys) feature for the Enhanced Object Literals section. The syntax highlighter seems to struggle a bit on those, sometimes the line is not showing up, so I had to re-indent the section (and [filed a bug](https://github.com/tmm1/pygments.rb/issues/115)).